Conservation of Energy ; 77 



93. It is easier to climb mountains in hobnailed shoes than in 



smooth-soled ones. 



94. When you bore a hole with a brace and bit, the hand that 



turns the brace goes around a circle many times as large 

 as the hole that is being bored. 



95. The hands of some persons become red and slightly swollen 



if they swing them while taking a long walk. 



96. A flywheel keeps an engine going between the strokes of the 



piston. 



97. In dry parts of the country farmers break up the surface of 



the soil frequently, as less water comes up to the surface 

 through pulverized soil than would come through the fine 

 pores of caked earth. 



98. After you have apparently cleaned a grease spot out of a suit 



it often reappears when you have worn the suit a few days. 



99. Mud flies up from the back wheel of a boy's bicycle when he 



rides along a wet street. 



100. A typewriter key goes down less than an inch, yet the type 

 bar goes up nearly 5 inches. 



SECTION 13. Action and reaction. 

 How can a bird fly? What makes it stay up in the air? 

 What makes a gun kick? 

 Why do you sink when you stop swimming ? 



Whenever anything moves, it pushes something else 

 in an opposite direction. When you row a boat you can 

 notice this ; you see the oars pushing the water back- 

 ward to push the boat forward. Also, when you shoot 

 a bullet forward you can feel the gun kick backward; 

 or when you pull down hard enough on a bar, your 

 body rises up and you chin yourself. But the law is 

 just as true for things which are not noticeable. When 

 you walk, your feet push back against the earth ; and 

 if the earth were not so enormous and you so small, 

 and if no one else were pushing in the opposite direction, 

 you would see the earth spin back a little for each step 



